While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.

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The production reality of sheet steels from casting to the end product is such that in the cases of ultra- and advanced high-strength steels, we have to deal with the segregation of elements on macro- and microlevels. Both can have a significant impact on the microstructure formation and resulting properties. There are several production stages where it can influence the transformations, i.e., casting, hot rolling process and annealing after cold rolling. In the present work, we focus on the latter, and more specifically, the transformation from ferrite–cementite to austenite, especially the nucleation process, in cold-rolled material. We vary the levels of two substitutional elements, Mn and Si, and then look in detail at the microsegregation and nucleation processes. The classical nucleation theory is used, and both the chemical driving force and strain energy are calculated for various scenarios. In the case of a high Mn and high Si concentration, the nucleation can thus be explained. In the cases of high Mn and low Si concentrations as well as low Mn alloys, more research is needed on the nuclei shapes and strain energy.@en

The HIsarna off-gas system wall is a cooling jacket made of cooling pipes arranged in the radial direction and in a circular pattern. Part of the off-gas system cooling pipes are isolated using a low-thermal-conductivity refractory material to protect the cooling pipe from melting and thermal stresses. During long runs and due to thermomechanical stresses, the refractory material is lost, and its thickness is reduced. It is possible to measure the thickness of the refractory layer only during shutdown, which is a disadvantage during long runs. The aim is to investigate the possibility of predicting the thickness of the refractory material by using other parameters that are possible to measure during the operation. A combination of FEM and CFD modeling is used to develop a methodology for detailed wall modeling and refractory material loss prediction. Finite element method (FEM) analysis is used to obtain the thermal properties of the wall using detailed geometries for variable refractory thickness. The obtained properties are then used to build CFD models to study the effect of refractory thickness on wall heat loss, temperature and composition profiles. The proposed procedure is validated against the plant measurement, and according to the findings, it is possible to relate the wall thickness to measured parameters such as heat loss through the walls, temperature and carbon conversion. @en

In this paper a CFD analysis of HIsarna off-gas system for post combustion of CO-H2-carbon particle mixture is presented to evaluate the effect of different sub-models and parameters on the accuracy of predictions and simulation time. The effects of different mesh type, mesh grid size, radiation models, turbulent models, kinetic mechanism, turbulence chemistry interaction models, including and excluding gas-solid reactions, number of reactive solid particles are investigated in detail. Based on the accuracy of the predictions and agreement with counterpart measured values, the best combination is selected and conclusions are derived. It was found that radiation and turbulence chemistry interaction model have a major effect on the temperature and composition profile prediction along the studied off-gas system, compared to the variations in other models. The effect of these two models becomes even more evident when the temperature and fuel content of the flue gas are high.@en

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

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Modelling dislocation glide over the initial part of a stress–strain curve of metals received little attention up to now. However, dislocation glide is essential to ones understanding of the fundamental relationship between inelastic deformation and the evolution of the dislocation network structure. Therefore, we present a model of dislocation-driven deformation under static loading conditions. We reproduce repeated cyclic uniaxial tensile tests on Interstitial-Free and Low-Alloy steels. The elastic mechanical behaviour is described by isotropic linear elasticity, pre-yield anelastic mechanical behaviour by a dislocation bow-out model with dissipation, and the post-yield evolution of dislocation network structure by a statistical storage model. We hypothesise that when the local anelastic compliance is lower than the global plastic compliance, deformation is mechanically recoverable, and vice versa. This hypothesis is corroborated with the classical Taylor relation. We report the relation between stable and unstable dislocation glide using this prototypical modelling framework. We find four structural variables, that are based on dislocation physics, to describe the stress–strain curve: total dislocation density, average dislocation segment length, dislocation junction formation rate, and average dislocation junction length. Firstly, we quantify the dislocation network evolution during uniaxial monotonic loading, and verify work-hardening by dislocation junction formation and a Taylor-type equation for flow. Finally, we present a semi-empirical relation for the evolution of the dislocation network structure. Which allows us to: refine the physical interpretation of the Taylor relationship, and rationalise experimental observations on apparent modulus degradation by thermomechanical processing. Both these findings circumvent the limitations of current, physics-based hardening models.

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A three-dimensional computational fluid dynamics (CFD) model for the HIsarna off-gas system is set up and validated by real plant data. In the model detailed reaction mechanism and kinetic data for post-combustion of CO-H2 mixture and carbon particles are incorporated. The results are presented and discussed in another study (Part 1) by the same authors. In the present paper, the focus will be on geometry modification of the off-gas system and the effects on the operating parameters. The effect of this modification on heat loss, temperature profile, carbon conversion and gaseous phase composition across the off-gas system is investigated. It is shown that the modified geometry leads to a higher heat loss through the reflux chamber walls which can change the temperature profile and consequently species composition. The modified geometry also offers possibility of higher CO-H2 mixture and carbon particles conversion rate and reduce unwanted emission from the reflux chamber.

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The HIsarna process is a new and breakthrough smelting reduction process for hot metal (liquid iron) production from iron ores and coal directly fed into the reactor. The flue gas from the main reactor enters the off-gas system containing small amounts of H₂, CO and carbon particles which need to be removed before further treatment by post combustion oxygen injection. A three-dimensional Computational Fluid Dynamics (CFD) simulation of the HIsarna off-gas system is performed and validated using a detailed reaction mechanism and kinetic data for post-combustion of a CO–H₂ mixture and carbon particles. Using the validated model, a series of simulations were performed to investigate the effect of water quenching and post combustion oxygen injection. It was found that water quenching can significantly reduce the off-gas temperature. It is also possible to reduce oxygen injection during operations where inlet CO content of the off-gas system is low.

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For all industrial applications, predicting system characteristics and behavior plays a vital role before constructing costly and complex multi-physic systems. Correct and reliable predictions become even more important once the aim is to go from small- to large-scale processes to establish an industrial demonstrations. In this study, a CFD-based scale-up of HIsarna off-gas system based on the Eulerian–Lagrangian approach is investigated and detailed step in scale-up procedure is discussed. A three-dimensional CFD model is developed and validated based on the available pilot scale data and used to design and scale up the post-combustion chamber (also known as reflux chamber). Detailed kinetics for volumetric and gas–solid reactions are incorporated in validated CFD model with a special attention to the wall boundary condition and modeling. The effect of reflux chamber geometry, oxygen injection ports, oxygen injection flowrate, isolation wall thickness, and inlet flue gas composition on different system characteristics such as heat loss through the wall, CO–H₂–carbon mixture conversion, flue gas, and wall temperature are investigated. The aim of the scaled up geometry, like pilot scale, is to achieve full combustion of unwanted species inside the reflux chamber to assure zero emissions from the off-gas system. Compared to the pilot scale, the scaled up reflux chamber is capable of handling and removing higher amount of unwanted species coming from the main reactor and therefore lower CO–H₂ and carbon particle emissions, mainly due to a larger size which provides larger volume and residence time for volumetric and gas–solid reaction to proceed.

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Within the steelmaking industry, a large amount of zinc-bearing waste is produced which cannot be effectively treated through integrated steel mills. Concurrently, zinc smelters generate waste residues containing significant amounts of iron and zinc which are stored or landfilled. The zinc concentration of iron and steelmaking residues inhibits its recycling to the blast furnace but is insufficient to be sent directly to the zinc producers. Consequently, a means of up-concentration is required. The pilot HIsarna ironmaking furnace has shown potential for processing secondary iron-bearing resources. Furthermore, zinc can be concentrated in the off-gas flue dust, providing an enriched input for zinc smelters. The potential recyclability of blast furnace (BF) and basic oxygen furnace (BOF) dust and ‘goethite’ residue from the zinc industry has been studied. The input materials have been comprehensively characterized and their reduction–vaporization behavior, has been investigated. Individual samples were tested at temperatures of up to 1300 °C. Here, it was shown that minimal reduction of iron and volatilization of zinc occurred in the goethite and BOF samples. Conversely, even at 1000 °C, the BF dust showed complete reduction of iron and removal of zinc within 30 min. This was due to its high carbon content (40 wt%) which can act as a reductant. Consequently, mixtures of BOF dust and goethite with BF dust were studied. It has been shown that mixtures of 30:70 BF dust to goethite and 20:80 BF dust to BOF dust are suitable for recovering zinc to the gas phase and fully reducing the contained iron. Graphical Abstract: [Figure not available: see fulltext.]

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HIsarna is a novel ironmaking process with great raw materials versatility that is attractive for various secondary resources. Among the materials that can be recycled, there is steel scrap which is fed to the furnace bath through an inclined chute. The velocity distribution of the scrap particles along the chute affects the particles’ distribution on the liquid slag and, thereupon, the efficient operation of the reactor. In this study, the flow of steel scrap particles along an inclined chute with the same dimensions as those of the actual chute of the HIsarna plant is investigated experimentally and numerically. The simulations are validated using chute tip velocity and mass fractions collected at the different compartments of a sampling device. Translational and angular velocity distributions along and across the chute are reported, and the effect of different parameters are investigated. The impact of the shape of the particles on the simulation process is found to be negligible. The angular velocity distribution in cross-sections of the chute exhibited a V-shaped orientation, whereas the translational velocity displayed similar values across the cross-sections. Moreover, translational velocity appeared to increase with increasing inclination angles, whereas angular velocity increased with decreasing batch size.

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HIsarna reactor is characterized by a high raw materials versatility and is therefore attractive for processing secondary iron sources. Among the materials that can be recycled through HIsarna, zinc-bearing material has drawn a special attention. Based on the plant data, once dust-containing Zinc was injected into the main reactor, a final collected dust with a zinc content of 16% was achieved which opened up possibilities of higher enrichment for direct reuse in Zn smelting as a secondary source and an alternative for Zn ore (the primary source). However Zn vapor can react with iron oxide to form zinc ferrite (ZnFe₂O₄), which is an undesired product. Hence, the main focus of this study is to minimize the formation of ZnFe₂O using thermodynamic (FactSage) and computational fluid dynamic tools. After detecting regions with high potential of ZnFe₂O₄ formation, proper geometrical and operational modifications of the off-gas system is proposed to minimize the formation of zinc ferrite.

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Galvanized steel scrap flow and injection into the HIsarna reactor are investigated using discrete element method (DEM). The scrap particle is fed into the reactor through an inclined chute and hits the slag surface where the zinc content is evaporated and solid particles melt. A DEM model is setup and validated using experimental data obtained from the exact plant-scale chute geometry and scrap particles. Using the DEM model, the effect of chute inclination, injection elevation, injection mode (batch and continuous), batch size, and flowrate on particle distribution and exerted pressure on the slag surface are investigated. It is found that continuous mode of injection is the most suitable method to increase the spread of particles and also to reduce the exerted pressure on the slag surface. Placing dent-like obstacles at the tip of the chute significantly increases the impact area, especially for batchwise injection, thus reducing force and pressure on the slag surface that minimizes the risk of liquid splash. Larger particle impact area is also beneficial to obtain higher zinc evaporation rate from particle surface and also to minimize the slag surface temperature disturbance.

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In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.

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The circular economy demands waste utilization for the production of high-value products, and this requires the development of novel processing routes. In this study, rare earth (La and Ce) oxides were completely (>99%) recovered from polishing waste by a combined novel reductive acid leaching and alkali treatment process. About 70% of rare earths were dissolved during the first leaching step. The undissolved rare earth compounds are converted to oxides/hydroxides by alkali treatment and dissolved in the acid solution – the 2nd leaching step – for the complete recovery of rare earths. The recovered rare earth oxides were used for producing in-situ high-value Al-La-Ce alloys with fused salt electrolysis. Mechanical properties of our Al-La-Ce alloys are similar to the known high temperature Al-Ce alloys. This development of new alloys by our novel process helps in utilization of both overproduced primary La and Ce oxides as well as La and Ce recovered from polishing waste.

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Intricate knowledge of dislocation networks in metals has proven paramount in understanding the constitutive behaviour of these materials but current experimental methods yield limited information on the characteristics of these networks. Recently, the isotropic anelastic response of metals has been used to investigate complex dislocation networks through the well-known phenomenon that the observed elastic constants are influenced by dislocations. Considering the dependence of the behaviour of a Frank-Read (FR) source on its initial dislocation character and using discerning characteristics of dislocations, i.e. Burgers vector, line sense and slip system, the present paper takes dislocation character, crystal structure and dislocation network geometry into account and obtains the anisotropic mechanical response for a generic Poisson's ratio. In this work, the tensile test tangent moduli and yield points are presented for spatially uniform and nonuniform dislocation distributions across slip systems. First, the reversible shear strain of the FR source is derived as a function of initial dislocation character. The area swept by a mobile and initially straight dislocation segment pinned at both ends is given as an explicit function of the line stress. Secondly, the anisotropic anelastic strain contribution of FR sources to the total pre- and at-yield strain in single crystallites is calculated. For a given normal stress and superposition of the principal infinitesimal linear elastic lattice strain and anelastic dislocation strain, the tangent moduli are presented. The moduli and the inception of plastic flow have a notable dependence on initial dislocation character, spatial dislocation distribution and loading direction.

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Interphase precipitation occurring during solid-state phase transformations in micro-alloyed steels is generally studied through transmission electron microscopy, atom probe tomography, and ex situ measurements of Small-Angle Neutron Scattering (SANS). The advantage of SANS over the other two characterization techniques is that SANS allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting matrix at room temperature. However, the performance of ex situ SANS measurements alone does not provide information regarding the probable correlation between interphase precipitation and phase transformations. This limitation makes it necessary to perform in situ and simultaneous studies on precipitation and phase transformations in order to gain an in-depth understanding of the nucleation and growth of precipitates in relation to the evolution of austenite decomposition at high temperatures. A furnace is, thus, designed and developed for such in situ studies in which SANS measurements can be simultaneously performed with neutron diffraction measurements during the application of high-temperature thermal treatments. The furnace is capable of carrying out thermal treatments involving fast heating and cooling as well as high operation temperatures (up to 1200 °C) for a long period of time with accurate temperature control in a protective atmosphere and in a magnetic field of up to 1.5 T. The characteristics of this furnace give the possibility of developing new research studies for better insight of the relationship between phase transformations and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features.

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In-situ Small-Angle Neutron Scattering (SANS) is used to determine the time evolution of the chemical composition of precipitates at 650 °C and 700 °C in three micro-alloyed steels with different vanadium (V) and carbon (C) concentrations. Precipitates with a distribution of substoichiometric carbon-to-metal ratios are measured in all steels. The precipitates are initially metastable with a high iron (Fe) content, which is gradually being substituted by vanadium during isothermal annealing. Eventually a plateau in the composition of the precipitate phase is reached. Faster changes in the precipitate chemical composition are observed at the higher temperature in all steels because of the faster vanadium diffusion at 700 °C. At both temperatures, the addition of more vanadium and more carbon to the steel has an accelerating effect on the evolution of the precipitate composition as a result of a higher driving force for precipitation. Addition of vanadium to the nominal composition of the steel leads to more vanadium rich precipitates, with less iron and a smaller carbon-to-metal ratio. Atom Probe Tomography (APT) shows the presence of precipitates with a distribution of carbon-to-metal ratios, ranging from 0.75 to 1, after 10 h of annealing at 650 °C or 700 °C in all steels. These experimental results are coupled to ThermoCalc equilibrium calculations and literature findings to support the Small-Angle Neutron Scattering results.

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This study presents the effects of silicon (Si) and manganese (Mn) concentration and of heating rate on the ferrite recrystallization kinetics in seven model alloys with different Si and Mn concentrations, which are of relevance for the development of Advanced High Strength Steels (AHSS). The recrystallization kinetics were studied with in-situ 2D X-ray Diffraction (2D-XRD) and ex-situ microstructure observations using Scanning Electron Microscopy (SEM). The experimentally observed differences in the recrystallization start temperature (Ts), dependent on the Si and Mn concentrations and the heating rate, can be described by combining the non-isothermal JMAK-model with a modified version of Cahn's solute drag model. The modified Cahn model takes into account - in an approximate manner - that the interaction energy of the solute atoms with the grain boundary depends on the Si and Mn concentrations in the boundary and the Wagner interaction parameters. The collective contribution of the Si and Mn atoms to the increase in the Ts with respect to the reference alloy (without Si and with very little Mn) is higher than would be expected from the simple addition of the effects of the Si and Mn concentrations alone. This means that the interaction between Si and Mn atoms leads to an additional increase in Ts, i.e. a coupled solute drag effect. For the later stages of recrystallization, we have studied the change in the number density and the growth rates of the recrystallizing grains using SEM. The observations show non-random nucleation, early impingement of the grains in the normal-direction and non-constant growth rates of recrystallizing grains.

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The steelmaking industry produces large quantities of zinc-bearing wastes of varying forms that cannot be treated through integrated steelmaking processes. Simultaneously, by-products of the zinc industry containing great amounts of iron and zinc are stored or landfilled. The amount of zinc in these materials is generally below that which is of value to be recycled directly to the zinc smelter, consequently a method of concentration is required. Tata Steel owns and operates the pilot HIsarna ironmaking plant which, due to its high raw materials flexibility, is attractive for the purpose of processing secondary iron sources. Furthermore, it can facilitate the simultaneous recovery of a zinc-enriched flue dust. The high temperature behaviour of various waste materials will be presented with regards to their recyclability in the HIsarna furnace. Blast furnace (BF) sludge and basic oxygen furnace (BOF) sludge from Tata Steel IJmuiden have been studied along with ‘goethite’ waste produced by Nyrstar. The various input materials have been comprehensively characterised and their reduction/vaporisation behaviour recorded. Mixed samples have been produced and tested in order to define the most appropriate form of delivery of these materials to the HIsarna furnace.

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